1. Field of the Invention
The present invention relates to an optical film used in various image display apparatus such as liquid crystal displays (LCDs). More specifically, the present invention relates to a laminated polarizing plate with a brightness enhancement film and various image display apparatus using the same.
2. Description of Related Art
Conventionally, polarizing plates often have been used in liquid crystal displays, and the demand therefor has been increasing rapidly. Furthermore, in recent years, high-value-added polarizing plates such as those having an optically compensating function have come into use. There has been expanding expectations for better display quality in terms of, for example, hue, brightness, contrast and viewing angle.
For the purpose of enhancing brightness among the above, a brightness enhancement film is used together with the polarizing plate. The brightness enhancement film usually is arranged between a backlight unit and a polarizing film in a liquid crystal display, and reflects and reuses light that is otherwise absorbed in the polarizing film, thereby enhancing the brightness of a display screen. As such a brightness enhancement film, for example, a laminate of polymer films having birefringence, a cholesteric liquid crystal film, etc. are known. In general, the laminate of polymer films reflects linearly polarized light, while the cholesteric liquid crystal film reflects circularly polarized light.
The cholesteric liquid crystal film is used, for example, as a laminate with a quarter wavelength plate, which is a retardation plate. With this laminate, the cholesteric liquid crystal film first separates light from a backlight into right-circularly polarized light and left-circularly polarized light and transmits one of these circularly polarized lights, and the laminated quarter wavelength plate converts it into linearly polarized light, thus supplying it to the polarizing plate. On the other hand, the light reflected by the cholesteric liquid crystal film is led to, for example, a reflection plate disposed at the back of the backlight where its polarization state is changed, returns to the cholesteric liquid crystal film so as to be separated further (see JP 11(1999)-248941 A, for example).
As described above, by combining the brightness enhancement film with the polarizing plate, it is possible to enhance brightness characteristics of the liquid crystal display. However, there are problems as well, which will be described below.
The polarizing plate, generally called an absorptive dichroic polarizing plate, is usually produced by allowing a polyvinyl alcohol (in the following, abbreviated as “PVA”) film to adsorb iodine or a dichroic dye, followed by stretching to prepare a polarizing film, and then laminating protective films such as triacetylcellulose (TAC) on both surfaces of this polarizing film. Because of this stretching process, a tensile stress remains inside the polarizing film. It is known that this stress causes shrinkage of the film under a heating or high-temperature humidifying condition. The brightness enhancement film is laminated on the backlight side of the polarizing plate. Therefore, an increase in temperature inside a liquid crystal panel, for example, causes the shrinkage of the polarizing plate, and this shrinkage force acts on the brightness enhancement film, so that changes in optical characteristics such as a retardation change occur in a retardation plate (for example, the quarter wavelength plate) serving as a constituent member of the brightness enhancement film. The changes in optical characteristics of the retardation plate bring about changes in the property of the brightness enhancement film. As a result, the problem arises in that the brightness, hue and chromaticity variations occur in the display screen of the liquid crystal display.
In order to solve such a problem, (1) a method of reducing changes in dimension of the polarizing plate and (2) a method of interposing between the polarizing plate and the brightness enhancement film a layer for alleviating the shrinkage force acting on the brightness enhancement film can be considered, for example. However, even after the changes in dimension of the polarizing plate are reduced, the problem of retardation change cannot be solved if the dimension of the retardation plate changes considerably. Also, interposing the other layer goes against the need for lighter weight and thinner apparatus in recent years. On the other hand, not only the dimensional change of the polarizing plate causes the problem, but also the brightness enhancement film itself may be affected by the heating or high-temperature humidifying.